The standard of the Advanced Television System Committee (ATSC) suggests to use a signal obtained by modulating 12 independent data streams, which are trellis encoded and time-multiplexed, into 10.76 MHz-rate 8-level Vestigial Side Band (VSB) to transmit High Definition Television (HDTV) broadcasting through a terrestrial broadcasting channel. The frequency band of the signal is transformed into a frequency band of 6 MHz which corresponds to a standard Very High Frequency (VHF) or Ultrahigh Frequency (UHF) terrestrial television channel. Signals of the corresponding channel are broadcasted at a data rate of 19.39 Mbps per second. Detailed technology on the ATSC DTV standards and A/53 are readily available through the ATSC.
However, transmission signals of a conventional 8-VSB transceiver are distorted in indoor and mobile channel environments due to variable channel and multipath phenomena, and this degrades reception performance of the receiver.
In other words, transmitted data are affected by various channel distortion factors. The channel distortion factors include a multipath phenomenon, frequency offset, phase jitter and the like. To compensate for the signal distortion caused by the channel distortion factors, a training data sequence is transmitted every 24.2 ms, but a change in multipath characteristics and Doppler interference exist even in the time interval of 24.2 ms that the training data sequences are transmitted. Since an equalizer of the receiver does not have a convergence speed fast enough to compensate for the distortion of receiving signals, which occurs by the change in multipath characteristics and the Doppler interference, the receiver cannot perform equalization precisely.
For this reason, the broadcasting program reception performance of 8-VSB DTV broadcast is lower than that of an analog broadcast and reception is impossible in a mobile receiver. Even if reception is possible, there is a problem that a signal-to-noise ratio (SNR) satisfying Threshold of Visibility (TOV) increases.
In order to resolve the above problem, a dual stream transmission method where a DTV transmission signal is transmitted by separating general data and robust data is discussed recently. In other words, researchers seek to improve the performance of a DTV receiver by adding robust data which are less sensitive to external interruption to general data and transmitting the robust and general data together.
FIG. 1 is a block diagram showing a conventional DTV transmitter. As shown, the transmitter 100 includes: a first multiplexer 101, a data randomizer 103, a Reed Solomon (RS) encoder 105, a robust interleaver/packet formatter 107, a data interleaver 109, a robust encoder 111, a robust data processor 113, a trellis encoder 115, a second multiplexer 117, and a pilot adder/modulator/Radio Frequency (RF) converter 119.
The first multiplexer 101 multiplexes, a general data packet 121 and a robust data packet 123.
The general data packet 121 and the robust data packet 123 are serial data streams formed of 188-byte Moving Picture Experts Group (MPEG) compatible data packets, and they are inputted into and randomized in the randomizer 103 and 20-byte parity information is added thereto for Forward Error Correction (FEC) in the RS encoder 105.
Subsequently, the robust interleaver/packet formatter 107 performs interleaving on the robust data and secures space for inserting a robust data header and a parity bit.
The packet-formatted robust data and the RS-encoded general data are interleaved in the data interleaver 109 and inputted into the robust encoder 111.
The robust encoder 111 and the trellis encoder 115 maps the inputted general and robust data to any one symbol level among {−7,−5,−3,−1,1,3,5,7}.
Meanwhile, the robust data processor 113 adds 20 parity bytes by performing RS encoding on the packet-formatted robust data to maintain low-rank compatibility with a conventional DTV receiver that does not support robust data.
The trellis-encoded general and robust data are combined with segment synchronization and field synchronization bit sequences from a synchronization unit (not shown) in the second multiplexer 117 to thereby generate a transmission data frame. Subsequently, a pilot signal is added thereto in a pilot adder. A symbol stream is modulated into VSB-suppressed carrier wave in a VSB modulator. A base-band 8-VSB symbol stream is finally converted into an RF signal in an RF converter.
The DTV receiver restores an MPEG data stream by carrying out a process reverse to the process of the transmitter on the transmitted signal.
A receiver that supports only general data can maintain backward compatibility by processing inputted robust data as null packets.
Also, a receiver capable of robust data can improve performance in receiving general and robust data entirely by raising a convergence speed of an equalizer and improving the performance of a trellis decoder.
As described above, fine reception performance can be expected even in a poor transmission environment by mixing general data with robust data and transmitting them in the form of dual stream.
However, the above-described DTV transmission system uses the parity bytes, which are obtained by performing RS coding on robust data and added, only for the backward compatibility of a low-ranked receiver and it does not use them for the purpose of error correction.
If the parity bytes added to the robust data can be used for the error correction, the robust data reception performance can be improved further.